This invention relates to a method for the removal of mercury present in exhaust gas, and more particularly to an exhaust gas treatment process which, especially when applied to a system for the desulfurization of exhaust gas discharged in huge amounts, can remove metallic mercury vapor effectively from the exhaust gas.
Harmful trace substances such as mercury are present in exhaust gas discharged from coal-fired and oil-fired plants, and it is difficult to remove them by the existing exhaust gas treatment systems. It is believed that mercury is present in exhaust gas, primarily in the form of metallic mercury (Hg) and mercury chloride (HgCl2). Since HgCl2 is easily absorbed into water, it can be removed in a desulfurizing absorption tower or the like. However, metallic mercury (Hg) having vary low solubility in water is, not absorbed in the desulfurizing absorption tower and may be discharged from a stack as metallic mercury vapor. At present, the amount of Hg which may be discharged from a stack is very small and has little influence on the environment. However, it is ideally preferable to eliminate such a risk completely.
To this end, the activated carbon adsorption technique, the sodium hypochlorite absorption technique and the like are being employed as Hg removal techniques.
With respect to the activated carbon adsorption technique, a method wherein a powder of activated carbon is blown into exhaust gas and recovered with a bag filter has already been put to practical use. However, this method is chiefly designed to treat exhaust gas resulting from garbage incineration, and no method suitable for use with a large-volume gas such as exhaust gas from an electric power plant is not known.
With respect to the sodium hypochlorite absorption technique, there are known, for example, methods wherein an additive such as sodium hypochlorite is directly added to cooling water for a cooling tower, an absorbing fluid within a desulfurizing absorption tower, or feed water or circulating water for a wet electrostatic precipitator. However, all of these methods involve the addition of an additive to the main apparatus of an exhaust gas treatment plant, and there is a possibility that the additive may impair their essential functions. For example, since the cooling tower has a low pH, a large amount of an oxidizing agent is required. Moreover, these methods are chiefly designed to treat exhaust gas resulting from garbage incineration, and are not suitable for use with a large-volume gas such as exhaust gas from an electric power plant.
On the other hand, another method for the removal of mercury present in exhaust gas is conceivable. According to this method, a mist eliminator is installed on the downstream side of a desulfurizer, and a solution containing an oxidizing agent (e.g., sodium hypochlorite) is sprayed into exhaust gas at a position upstream of the mist eliminator so as to oxidize the mercury.
However, this method has a problem in that, since an oxidizing agent is added to exhaust gas, the absorption of SO2 causes a large amount of the oxidizing agent to be consumed by reaction with SO2. Consequently, the sprayed oxidizing agent (e.g., sodium hypochlorite) scarcely remains and does not serve to recover mercury effectively, so that it becomes necessary to add sodium hypochlorite in quick succession. Since this causes an increase in the operating cost of the system, the aforesaid method has the disadvantage of being inefficient.
That is, even though sodium hypochlorite or the like is added as an oxidizing agent for the oxidation of mercury, the oxidizing agent fails to exhibit its effect unless it is fed in large amounts. Actually, the absorption of sulfur dioxide (SO2) causes the oxidizing agent to be consumed in an amount equal to or greater than the amount of the existing SO2, thus leading to an increase in operating cost.
In view of the above-described problems, the present inventors made intensive investigations for the purpose of developing an exhaust gas treatment process which can remove mercury, particularly metallic mercury vapor, from a large-volume gas (e.g., exhaust gas from an electric power plant) and in which the oxidative removal of mercury with the aid of an oxidizing agent can be effectively carried out even in the presence of SO2.
As a result, the present inventors have now found that, by adding a specific antioxidant to a solution of an oxidizing agent, the oxidizing agent can be effectively used for the removal of mercury, and the above-described problems can be solved thereby. The present invention has been completed from this point of view.
Specifically, the mercury removal method of the present invention is characterized in that, in an exhaust gas treatment process wherein exhaust gas containing mercury and sulfur dioxide is subjected to a desulfurization treatment in a desulfurizing absorption tower and then passed through a mist eliminator for removing and recovering mist from the desulfurized exhaust gas, an oxidizing agent and a specific antioxidant unreactive with the oxidizing agent are added to the desulfurized exhaust gas at a position upstream of the aforesaid mist eliminator.
By adding the antioxidant so as to allow the oxidizing agent to act effectively, the mercury-absorbing ability is enhanced and the operating cost is reduced. Moreover, by adding the oxidizing agent to the desulfurized exhaust gas at a position upstream of the mist eliminator, the amount of oxidizing agent fed may be reduced to a level of {fraction (1/10)} to {fraction (1/20)} as compared with the method in which an oxidizing agent is directly added, for example, to the absorbing fluid within the absorption tower. Furthermore, mercury can be removed from the desulfurized exhaust gas without contaminating other apparatus with the oxidizing agent.
The aforesaid antioxidant preferably comprises an alcohol, and the aforesaid oxidizing agent preferably comprises sodium hypochlorite, sodium chlorate or potassium permanganate.
In the mercury removal method of the present invention, it is a preferred embodiment to feed the aforesaid oxidizing agent and antioxidant to the desulfurized exhaust gas by spraying a solution containing them (i.e., an absorbing solution) into the desulfurized exhaust gas. It is especially preferable that the solution used for this purpose have a pH of 5 to 7.
As the aforesaid mist eliminator, there may be used, for example, a vertically or obliquely installed mist eliminator. Moreover, the aforesaid oxidizing agent and antioxidant may be fed cocurrently or countercurrently to the flow of the aforesaid desulfurized exhaust gas.
When an oxidizing agent is added to exhaust gas to be treated according to the present invention, the oxidation of sulfur dioxide constituting a component other than mercury proceeds. This oxidation reaction of sulfur dioxide (SO2) consists of an absorption reaction, an initiation reaction, a propagation reaction and a termination reaction, and proceeds as a chain reaction in which a radical (.SO3) is formed as an intermediate by the initiation reaction.
Since this radical has very high reactivity, it reacts rapidly with an antioxidant (i.e., a radical scavenger such as an alcohol capable of capturing radicals) added according to the present invention. Consequently, the oxidation reaction of sulfur dioxide can be stopped by reacting the exhaust gas with the radical scavenger prior to its reaction with the oxidizing agent such as hypochlorous acid.
Thus, the present invention enables an oxidizing agent to act effectively by the addition of an antioxidant, so that the mercury-absorbing ability can be enhanced and the operating cost of the system can be reduced. Moreover, the reduction in pH of an absorbing solution (i.e., a solution of the oxidizing agent) due to the absorption of sulfur dioxide is suppressed, so that material corrosion can be reduced and an adverse influence on the materials of piping and downstream equipment can be avoided.
The present invention is more fully described hereinbelow in connection with several specific embodiments thereof. However, it is to be understood that the present invention is not limited to these specific embodiments.